The rocks in these three photographs don’t look like they have much in common. I mean, they are all big. Really big. And they do all look like they were dropped onto unsuspecting landscapes by geologically-minded aliens. In reality, there’s more to these boulders than meets the eye. In a geologists’ dictionary, all of these boulders belong to one family – a class known as erratics.

What are erratics? Glad you asked. The word comes from the Latin ‘errare’, meaning to err or go astray. Quite literally, you can think of erratics as lost rocks. And lost they are. Each of the boulders above was carried to its present location by an ice sheet during the last glaciation, which ended approximately 10,000 years ago.

The climate then was much different than the climate of today. Much of Canada and the northern US was overlain by giant ice sheets, up to 3 km (2 miles) in thickness. Large boulders could be plucked by a glacier at its base or tumble onto the glacier during rockfalls. As the ice moved, the rocks would be carried with it, often hundreds or even thousands of kilometres away their source. As the climate warmed and glaciers began to melt, the rocks would be dropped in place or carried along within icebergs or on ice ‘rafts’. Eventually, the erratics would find their new home, in environments far different from those of their birth.

Today, erratics serve as one of many clues left behind by glaciers that allow geologists to reconstruct their histories. Every rock has a mineral ‘fingerprint’ that allows it to be matched to its source. By comparing the distribution of sources and erratic locations, we can reconstruct the movement of ice sheets and learn much about the climate and history of the Earth during the last ice age. And learn we have. Only two centuries ago, it was widely believed that erratics were borne and deposited in the torrents of the biblical flood. Today, we know that the last ice age lasted almost 100,000 years, during which time so much water was stored in ice and global sea level fell so low that primitive humans were able to cross on foot from Siberia into Alaska and make their way south through ice-free corridors. Now that is some cool history.

Until next time, be warned: you may have just sustained a lethal dose of mostly harmlessscience.

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